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Biomedical subjects

D A Melton

Publications and source records attributed to D A Melton.

At least 19 recordsLinked to original sources
PubMed1992

A truncated activin receptor inhibits mesoderm induction and formation of axial structures in Xenopus embryos.

Activins can induce mesoderm in embryonic explants and have been proposed as the natural inducer in Xenopus. A mutant activin receptor that inhibits activin signalling is used to show that activin is required for the induction of mesoderm in vivo and the patterning of the embryonic body plan. Blocking the activin signal transduction pathway also reveals autonomous induction of a neural marker and unmasks a relationship between activin and fibroblast growth factor.

Activin Receptors↗
PubMed1992

Specificity of antisense oligonucleotides in vivo.

Antisense oligonucleotides are widely used as inhibitors of gene expression in cultured cells and have been proposed as potential therapeutic agents, but it is not known to what extent they are specific for their intended target RNAs. Statistical considerations indicate that if oligonucleotides can form hybrids with mRNA molecules in vivo by means of short or imperfect regions of complementarity, then the specificity of oligonucleotides as antisense reagents will be greatly compromised. We have used Xenopus oocytes as a model system in which to investigate the potential specificity of antisense oligonucleotides in vivo. We injected perfect and partially matched antisense oligonucleotides into oocytes and measured the resulting degradation of the target RNA in each case. On the basis of the extent to which antisense oligonucleotides can cause cleavage of RNAs at imperfectly matched target sites, we conclude that in this system it is probably not possible to obtain specific cleavage of an intended target RNA without also causing at least the partial destruction of many nontargeted RNAs.

Animals↗
PubMed1992

Involvement of p21ras in Xenopus mesoderm induction.

During early vertebrate embryogenesis, mesoderm is specified by a signal emanating from prospective endoderm. This signal can respecify Xenopus prospective ectoderm as mesoderm, and can be mimicked by members of the fibroblast growth factor and transforming growth factor-beta families. In other systems, the p21c-ras proto-oncogene product has been implicated in signal transduction for various polypeptide growth factors. We report here that a dominant inhibitory ras mutant blocks the mesoderm-inducing activity of fibroblast growth factor and activin, as well as the endogenous inducing activity of prospective endoderm. A constitutively active ras mutant partially mimics these activities. These results indicate that p21ras may have a central role in the transduction of the mesoderm inductive signal. Basic fibroblast growth factor and activin have emerged as candidates for endogenous mesoderm-inducing molecules. The character of the mesoderm induced by these two factors is overlapping but distinct when assessed both by histological and molecular criteria. The signal transduction pathways used during induction by these factors are unknown. We used messenger RNA microinjection of Xenopus eggs to express a dominant inhibitory mutant ras, p21(Asn 17)Ha-ras, in cells competent to respond to inducing factors to examine the role of p21ras in this response. This mutant, which has a reduced affinity for GTP relative to GDP, blocks a variety of mitogenic signals in 3T3 fibroblasts as well as the differentiation of pheochromocytoma cells in response to nerve growth factor.

Activins↗
PubMed1992

Vegetal messenger RNA localization directed by a 340-nt RNA sequence element in Xenopus oocytes.

Contained within a single cell, the fertilized egg, is information that will ultimately specify the entire organism. During early embryonic cleavages, cells acquire distinct fates and their differences in developmental potential might be explained by localization of informational molecules in the egg. The mechanisms by which Vg1 RNA, a maternal mRNA, is translocated to the vegetal pole of Xenopus oocytes may indicate how developmental signals are localized. Data presented here show that a 340-nucleotide localization signal present in the 3' untranslated region of Vg1 RNA is sufficient to direct RNA localization to the vegetal pole.

Animals↗
PubMed1992

Embryonic expression and functional analysis of a Xenopus activin receptor.

We report the isolation and characterization of a Xenopus activin receptor (XAR1). The amino acid sequence of this protein shows extensive homology with a murine activin receptor. The mRNA is expressed maternally and is ubiquitously distributed during the early stages of embryogenesis. Consistent with a possible role in mesoderm induction and patterning, interference with the normal expression of the receptor by overexpression in the early embryo results in the formation of ectopic dorsal axial structures. During neurulation the XAR1 mRNA is expressed predominantly in the presumptive brain and spinal cord, suggesting an additional function for XAR1 in neurogenesis.

Activin Receptors↗
PubMed1992

Interaction of Wnt and activin in dorsal mesoderm induction in Xenopus.

Both the activin and Wnt families of peptide growth factors are capable of inducing dorsal mesoderm in Xenopus embryos. Presumptive ventral ectoderm cells isolated from embryos injected with Xwnt8 mRNA were cultured in the presence of activin A to study the possible interactions between these two classes of signaling proteins. We find that overexpression of Xwnt8 RNA alters the response of ventral ectoderm to activin such that ventral explants differentiate dorsoanterior structures including notochord and eyes. This response is similar to the response of dorsal ectoderm to activin alone. When embryos are irradiated with uv light to inhibit dorsal axis formation, ectodermal explants differentiate notochord when they are induced by a combination of both signaling factors, but not when cells receive only one inducing signal (activin or Xwnt8). This result is further supported by the observation that goosecoid (gsc) mRNA, an early marker for dorsal mesoderm, is expressed in these explants only when they are injected with Xwnt8 mRNA followed by exposure to activin. Early morphogenetic movements of the induced cells and activation of muscle-specific actin and Brachyury (Xbra) genes also reveal a cooperation of activin A and Xwnt8 in mesoderm induction.

Actins↗
PubMed1991

Injected Wnt RNA induces a complete body axis in Xenopus embryos.

Studies in Xenopus have shown that growth factors of the TGF beta and Wnt oncogene families can mimic aspects of dorsal axis formation. Here we directly compare the inductive properties of two Wnt proteins by injecting synthetic mRNA into developing embryos. The results show that Wnt-1 and Xwnt-8 can induce a new and complete dorsal axis and can rescue the development of axis-deficient, UV-irradiated embryos. In contrast, activin mRNA injection induces only a partial dorsal axis that lacks anterior structures. These studies demonstrate that the mechanism of Wnt-induced axis duplication results from the creation of an independent Spemann organizer. The relationship between the properties of the endogenous dorsal inducer and the effects of Wnts and activins is discussed.

Activins↗
PubMed1991

Pre-existent pattern in Xenopus animal pole cells revealed by induction with activin.

Activin, a peptide growth factor related to tumour growth factor-beta, has been implicated in early inductive interactions in vertebrates and can induce Xenopus blastula ectodermal explants to develop a rudimentary axial pattern with anteroposterior and dorsoventral polarity. Here we demonstrate that prospective dorsal and ventral regions of the ectoderm respond differently to the same concentration of activin. Thus, activin does not seem to endow ectodermal cells with polarity but rather reveals a pre-existent pattern. Our results suggest that patterning of mesoderm is determined not only by a localized inducer, but also by the differential competence of cells in the responding tissue.

Activins↗
PubMed1991

Pattern formation during animal development.

At the beginning of this century, embryologists defined the central problems of developmental biology that remain today. These questions include how differentiated cells arise and form tissues and organs and how pattern is generated. In short, how does an egg give rise to an adult? In recent years, the application of molecular biology to embryological problems has led to significant advances and recast old problems in molecular and cellular terms. Although not necessarily comprehensive, this idiosyncratic review is intended to highlight selected findings and indicate where there are important gaps in our knowledge for those less than familiar with developmental biology.

Activins↗
PubMed1991

The Xenopus localized messenger RNA An3 may encode an ATP-dependent RNA helicase.

The maternal messenger RNA An3 was originally identified localized to the animal hemisphere of Xenopus laevis oocytes, eggs and early embryos. Xenopus embryos depend on mRNA and protein present in the egg before fertilization (maternal molecules) to provide the information needed for early development. Localization of maternal mRNA gives cells derived from different regions of the egg distinctive capacities for protein synthesis. We show here that An3 mRNA encodes a protein with 74% identity to a protein encoded by the testes-specific mRNA PL10 found in mouse, which is proposed to have RNA helicase activity. Because the gene encoding An3 mRNA is reactivated after gastrulation and remains active throughout embryogenesis, we have examined its distribution in embryonic and adult tissues. Unlike PL10 mRNA, which is primarily restricted to the testes, An3 mRNA is broadly distributed in later development.

Amino Acid Sequence↗
PubMed1990

Activins are expressed early in Xenopus embryogenesis and can induce axial mesoderm and anterior structures.

We show that mammalian and Xenopus activins induce dorsal axial mesoderm and anterior structures in explants of Xenopus blastula cells that would otherwise form epidermis. The induced explants of animal cap cells can form notochord, muscle, neural tissue, and eyes all arranged in a rudimentary axial pattern. Activin A shares inductive properties and antigenic determinants with PIF, an inducing factor recently isolated from mouse macrophage culture supernatants. Genes encoding Xenopus activin beta A and beta B chains were cloned. Activin beta B transcripts are first detected in Xenopus blastula, whereas activin beta A transcripts do not appear until the late gastrula stage. Recombinant Xenopus activin beta B protein induces mesodermal and neural tissues similar to those induced by mammalian activin A and PIF. Furthermore, ectopic expression of Xenopus activin beta B produces a second body axis in embryos injected with synthetic mRNA. Our results suggest that early induction and axial patterning are accomplished by endogenous activin B, not activin A, in Xenopus.

Activins↗
PubMed1990

Activin can induce the formation of axial structures and is expressed in the hypoblast of the chick.

We show that PIF/activin can induce the formation of axial structures including a full-length notochord, segmented somites, and a neural tube in isolated epiblasts from chick blastulae. Using degenerate PCR primers, we have cloned a fragment of the activin beta B chain from chick hypoblast cDNA, and a fragment of the activin beta A chain from chick genomic DNA. Furthermore, we show that in the chick, activin is transcribed precisely when axial mesoderm is being induced. Since exogenous PIF/activin can induce the formation of axial structures and since activin beta B is transcribed at the time and place where the mesodermal axial structures are being induced, we propose that in the chick, activin B is the endogenous inducer of the body axis.

Activins↗
PubMed1990

A mouse macrophage factor induces head structures and organizes a body axis in Xenopus.

Soluble peptide factors have been implicated as the agents responsible for embryonic inductions in vertebrates. Here, a protein (PIF) secreted by a mouse macrophage cell line is shown to change the developmental fate of Xenopus embryonic cells. Exposure to PIF causes presumptive ectodermal explants to form anterior neural and mesodermal tissues, including brain and eye, instead of ciliated epidermis. In addition, the induced tissues are organized into a rudimentary embryonic axis. These results suggest that PIF or a closely related molecule is involved in inducing anterior structures and organizing the frog body plan.

Animals↗
PubMed1990

The stability, toxicity and effectiveness of unmodified and phosphorothioate antisense oligodeoxynucleotides in Xenopus oocytes and embryos.

The properties of antisense phosphorothioate and unmodified oligodeoxynucleotides have been studied in Xenopus oocytes and embryos. We find that phosphorothioates, like unmodified oligodeoxynucleotides, can degrade Vg1 mRNA in oocytes via an endogenous RNase H-like activity. In oocytes, phosphorothioate oligodeoxynucleotides are more stable than unmodified oligodeoxynucleotides and are more effective in degrading Vg1 mRNA. In embryos, neither unmodified nor phosphorothioate deoxyoligonucleotides were effective in degrading Vg1 message at sub-toxic doses.

Animals↗
PubMed1990

Identification of a novel transforming growth factor-beta (TGF-beta 5) mRNA in Xenopus laevis.

A novel transforming growth factor-beta (TGF-beta) mRNA of about 3.0 kilobases, which encodes a putative protein of 382 amino acids, has been identified in amphibians by cDNA cloning. This mRNA, which we designate as TGF-beta 5, is developmentally regulated and highly expressed beginning at early neurula (stage 14) and in many adult tissues in Xenopus laevis. Following the first methionine, the putative precursor protein has a hydrophobic region, approximately 22 amino acids long, which probably represents a signal sequence, similar to that found in TGF-beta s 1-3. The precursor also has potential sites for glycosylation, integrin binding (RGD), and a tetrabasic amino acid (RKKR) site for potential cleavage of the precursor peptide to a biologically active protein. The putative mature protein consists of 112 amino acids with 9 cysteines and has 76, 66, 69, and 72% identity to TGF-beta s 1-4, respectively.

Amino Acid Sequence↗